Polycarbonates are widely used polymers. Their popularity stems from an attractive set of mechanical and physical properties, such as impact resistance, heat resistance, and transparency. They can be made by a variety of techniques, such as direct phosgenation and ester interchange, which is often referred to as transesterification. Direct phosgenation can be carried out by an interfacial technique, which involves the reaction of an aromatic dihydroxy compound such as bisphenol A with phosgene.
The transesterification process has some definite advantages over direct phosgenation. First, it does not rely on the use of phosgene, a material which is toxic and somewhat difficult to handle. Moreover, transesterification usually does not result in the excessive generation of chloride ions, which sometimes occurs in phosgenation processes.
Transesterification involves reaction between an aromatic dihydroxy compound and a carbonic acid diester. It is usually carried out in the molten state, wherein a polycondensation reaction results in the production of the product polymer. Batch reactors may be used, but when large volumes are involved, continuous processes are usually employed. These processes often include one or more continuous-flow stirred-tank reactors (CSTR's), where oligomers of a low molecular weight and viscosity are made, followed by one or more finishing reactors, which convert the oligomers into high molecular weight polymer. The reaction requires at least one catalyst. Sometimes, a combination of catalysts is used, such as an alkali metal or alkaline earth metal compound, along with a quaternary ammonium compound.
Precise control of the catalyst level is often critical in the preparation of polycarbonates. This is especially true in the case of the alkali metal or alkaline earth metal catalysts, which are sometimes employed at the parts-per-billion to parts-per-million level. In practice, control of the "effective amount" of catalyst is actually the critical task, i.e., control of the amount which actually catalyzes the transesterification reaction, without consideration of the remainder of the catalyst. The remainder of the catalyst may be participating in a multitude of side reactions. For example, it may be neutralizing impurities or being quenched in some other way. Moreover, the impurities themselves could be catalyzing the primary transesterification reaction, e.g., if their net concentration is chemically basic. The impurities may inadvertently enter the reaction system by a wide variety of routes. For example, they may originate with the aromatic dihydroxy compound, or the carbonic acid diester. They may also leach into the reactor through numerous routes.
If too much "effective" catalyst is present in a continuous polymerization process, some serious problems may occur. For example, the molecular weight of the product may be excessive, and the product can exhibit increased yellowing. Moreover, undesirable side reactions may take place. Furthermore, residual catalyst which remains in the polymer product may cause the product to exhibit decreased hydrolytic stability.
At the other end of the spectrum, too low a level of effective catalyst can also present problems. The most serious consequence is a low molecular weight product which will not possess the mechanical properties required for many commercial applications.
Those of skill in the polymer arts--especially those who work with polycarbonates--must therefore acknowledge that a very accurate method for controlling the level of effective catalyst in a transesterification process would be a welcome advancement. The method should be completely compatible with the primary reaction being carried out to prepare the polymer. Moreover, the method should not require any costly additions to the overall process--in terms of both equipment and processing time. Finally, the polymer product resulting from catalyst adjustment according to this invention should possess a set of desirable physical and mechanical properties which one typically expects from polycarbonates.